The unpronounceable masterpiece of the Industrial Revolution: Telford’s Pontcysyllte Aqueduct.

Of all the stupendous engineering structures produced by the Industrial Revolution, this is one of the most extraordinary: a ribbon of water in the sky. A narrow cast-iron water-filled trough, over 1,000 feet long, strides out across a steep-sided Welsh valley on a series of slender stone piers. Canal boats drift across, reaching a height of 126 feet above the valley floor. I first made this trip as a child and it was exhilarating and terrifying at the same time. It still is. Because while there is a towpath and handrail on one side of you, on the other there is nothing but the thin lip of the trough, rising to only a few inches above the waterline. It does not look strong enough. You feel you are going to plunge over the edge.

This is one of those marvels of engineering and architecture that really should not exist. Economically, it never made any sense. It was a product of Britain’s canal-building mania of the 1790s, it opened in 1805, and found itself on a route that went nowhere much, and then stopped. Having been built, it should not logically have survived. It is sited on a truncated stretch of waterway, a puzzling fragment of a much larger, never-completed scheme. This was known as the Ellesmere Canal, intended to link the mighty rivers of Mersey and Severn via the coal and iron ore mines of North Wales. But no sooner had engineers Thomas Telford and William Jessop completed this hugely ambitious structure – along with other expensive aqueducts and tunnels, piercing the hills and leaping the valleys to get to this point – than financial reality took hold and the project was halted. Commercial boat traffic on the inconclusive sections that were built was always light, and had ceased altogether by 1939. The waterway was officially abandoned to navigation in 1944. But salvation was at hand.

It, and its matchless aqueduct, survived for two reasons. Almost by accident, it provided a fresh water supply from the Welsh hills to the towns and cities of north-west England. And it became an early campaign victory, a symbol, for Britain’s nascent waterways preservation movement in the 1940s. The canal network was being rediscovered by a generation of postwar nostalgists, alert both to industrial heritage and to the fast-vanishing gipsy-like lifestyle of the traditional boating families in their “narrow boats” (never called barges). This particular canal, branching off the main system and heading romantically into the hills, rapidly became the most popular of them all. It was rebranded as the Llangollen Canal after the little town where it finally peters out, but what drew people and boats to it was the Pontcysyllte Aqueduct. Long before the concept was formalised by marketeers, this had become a leisure destination in itself.

Today there is virtually no commercial traffic anywhere on the quaintly narrow-gauge British canal network, where most locks and bridges dictate a boat size of no more than seven feet wide and 70 feet long. On the other hand, leisure use is now huge. This would have mystified Telford and Jessop, Georgian engineers whose roads, docks, bridges, waterways and early railways were wholly industrial ventures. Jessop, himself a pioneer of iron construction, was the overseer for this route, with the younger Telford – son of a Scottish shepherd – in charge of detailed design. The aqueduct has always been ascribed to Telford, but we know that he turned to Jessop for advice and approval.

Consider: this is a 200-year-old prefabricated structure of supreme elegance and durability. The bolted cast-iron plates form sections of trough, 11 feet wide and over five feet deep. They were caulked at the joints with Welsh flannel steeped in boiling sugar, and then sealed with lead. All together, these comprise a long tank holding 400,000 galllons of water, supported on 19 arches – also in iron – each with a 45-foot span. It steps lightly across the valley of the River Dee on tapering stone piers which become hollow towards the top. What few boaters notice is the remarkably tall embankment – the most ambitious ever attempted at the time – which Telford built out from the valley side to approach it.

The structure and the landscape thus become as one, and the experience of moving slowly in a boat out from the shadows of the valley side and across the aqueduct, looking down at the Dee tumbling over rocks far below, approaches the sublime. Up there, so little separates water from air. Weight becomes weightless. Even at the time, it was acclaimed as a work of art as much as engineering.

The term “high-tech” tends to be loosely applied to works of architecture which like to show off their structural virtuosity, but which should more often be described as “industrial chic”. At Pontcysyllte – a name that means “the bridge that connects” – you get the real thing. Telford initially saw himself as an architect rather than as an engineer, and here the disciplines fuse perfectly.

Now it is rightly regarded as a prime industrial monument, and is likely to receive World Heritage status before long. But remember that this is not a ruin. Two centuries on, it still functions exactly as intended. It is still awe-inspiring. It was thoroughly restored for its bicentennial last November, but it had stoically survived decade after decade of neglect in the 20th century. Looking at it today, it is as miraculous a fusion of architecture, engineering and nature as ever. This is a work of prodigious genius.

Note: so how DO you pronounce Pontcysyllte? Aim at something like Pont-kersulty.

Text and photos © Hugh Pearman. First published in the Wall Street Journal’s “Masterpiece” series on February 4, 2006 as “A marvel that shouldn’t exist”.

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